Advice on keeping children safe when they travel:

A properly fitted child car seat will help to prevent your child from being thrown about inside the vehicle, or ejected from it, if there is a crash. It will also absorb some of the impact force, and provide some protection from objects intruding into the passenger compartment.

A seat belt on its own will not properly fit your child, until they are at least 135 cm (4'6") tall, although it's better to wait until they are 150 cm (5ft) or taller before moving them to the seat belt on its own.